<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .header{
            width: 200px;
            height: 200px;
            z-index: 1;
            background-color: rgb(0, 0, 0, 0.7);
            border-radius: 50%;
            background: linear-gradient(to bottom right, #68e4be , #9191bd);
        }
        .nav{
            position:absolute;
            top: 10px;
            left: 10px;
            z-index: 2;
            width: 180px;
            height: 180px;
            background-color: #FFF;
            border-radius: 50%;
            margin: 8px;
        }
    </style>
</head>
<body>
    <header class="header">
        <nav class="nav"></nav>
    </header>
</body>
</html>